Comparative study of medium-pore zeolites (ZSM-5, ZSM-23, ZSM-48) and SAPO-11 in the hydroisomerization of <i>n</i> -hexadecane

The high selectivity of bifunctional catalysts in the hydroisomerization of long-chain n-paraffins is a key factor for producing high-quality fuels. However, it is directly dependent on a complex balance between the topology, acidity, morphology, and crystal size of the zeolite support. To address this challenge, this work presents a comparative study of medium-pore molecular sieves (ZSM-5, ZSM-23, ZSM-48, and SAPO-11) with different structural and acidic properties. The physicochemical characteristics of the materials were investigated using a set of analytical techniques, including XRD, SEM, low-temperature N2 adsorption, and NH3-TPD. It was shown that the pore topology (3D in ZSM-5 vs. 1D in the others), chemical composition, and crystal morphology (ranging from ~50 nm nanoprisms for ZSM-48 to 200–300 nm needles for ZSM-23-80 and 1 μm needles for ZSM-23-100) determine both the textural properties and the strength and concentration of acid sites. In the hydroisomerization of n-hexadecane, the Pt/ZSM-5 catalyst, with its high concentration of strong acid sites and 3D channel pore structure, exhibited the highest activity but an extremely low isomer selectivity (7%) due to intense cracking. The best results were demonstrated by the Pt/SAPO-11 and Pt/ZSM-48 samples, which provided isomer yields of 80% and 73%, respectively. Their high efficiency is attributed to the combination of a one-dimensional pore structure, moderate acidity, and nanocrystalline size, which minimizes diffusion limitations and suppresses side reactions. Thus, controlling the topology and acidity in combination with nanocrystalline morphology is an effective strategy for the rational design of highly selective hydroisomerization catalysts.